1. Field of the Invention
The present invention relates to piconet wireless networks. More particularly, it relates to frequency compensation in radio frequency (RF) devices.
2. Background of Related Art
Piconets, or small wireless networks, are being formed by more and more devices in many homes and offices. In particular, a popular piconet standard is commonly referred to as a BLUETOOTH™ piconet. Piconet technology in general, and BLUETOOTH technology in particular, provides peer-to-peer communications over short distances. The BLUETOOTH™ specification Version 1.1 is available at www.bluetooth.com, and is explicitly incorporated herein by reference.
The wireless frequency of piconets may be 2.4 GHz as per BLUETOOTH standards, and/or typically have a 20 to 100 foot range. The piconet RF transmitter may operate in common frequencies which do not necessarily require a license from the regulating government authorities, e.g., the Federal Communications Commission (FCC) in the United States. Alternatively, the wireless communication can be accomplished with infrared (IR) transmitters and receivers, but this is less preferable because of the directional and visual problems often associated with IR systems.
A plurality of piconet networks may be interconnected through a scatternet connection, in accordance with BLUETOOTH protocols. BLUETOOTH network technology may be utilized to implement a wireless piconet network connection (including scatternet). The BLUETOOTH standard for wireless piconet networks is well known, and is available from many sources, e.g., from the web site www.bluetooth.com.
According to the BLUETOOTH specification, BLUETOOTH systems typically operate in a range of 2400 to 2483.5 MHz, with multiple RF channels. For instance, in the US, 79 RF channels are defined as f=2402+k MHz, k=0, . . . , 78. This corresponds to 1 MHz channel spacing, with a lower guard band (e.g., 2 MHz) and an upper guard band (e.g., 3.5 MHz).
The BLUETOOTH specification requires transmission using Gaussian Frequency Shift Keying (GFSK), with a binary one being represented by a positive frequency deviation, and a binary zero being represented by a negative frequency deviation. FIG. 4 shows the function of a conventional peak detector in determining a maximum positive peak offset frequency and a maximum negative peak offset frequency in a GFSK system. According to the BLUETOOTH piconet standard, the minimum deviation shall never be smaller than 115 KHz. Also, the transmitted initial center frequency accuracy must be +/−75 KHz from the desired center frequency.
All receiving devices have a local clock on which a baseband receive clock signal in an RF section is based. To receive a radio frequency (RF) signal from another piconet device, the receiving device must lock onto the transmitted frequency.
It is important to note that in the real world, clock signals jitter and vary somewhat within desired tolerable limits. Other than the frequency requirements, the BLUETOOTH standard specifies that the clock jitter (rms value) should not exceed 2 nS and the settling time should be within 250 uS. A significant source of clock variation is the variance between external crystal oscillators installed in any particular BLUETOOTH device. Temperature also causes variations in clock signals.
In the RF transceiver of a BLUETOOTH conforming device, the alignment of a local oscillation (LO) with a received RF signal is important to guarantee proper and correct receipt of the underlying data signal. Thus, to eliminate any frequency offset of the received RF signal with respect to the local oscillation signal, automatic frequency compensation is employed.
Automatic frequency compensation (AFC) estimates frequency offset, and appropriately adjusts the local oscillation signal, typically via a voltage controlled oscillator (VCO). In BLUETOOTH applications, because of the tight schedule of receive timing, the AFC is divided into two stages: (1) course-AFC; and (2) fine-AFC.
Course-AFC is performed during the short period when the data frame begins. During course-AFC, the receiving device roughly adjusts its local oscillation so that the frequency offset becomes smaller, e.g., allowing for the correct recognition of an appropriate access code. Thereafter, fine-AFC is performed until the completion of the data frame. During fine-AFC, the frequency offset is adjusted to a minimum value such that the frequency offset does not affect the bit error rate (BER) of the received signal.
Conventional automatic frequency compensation uses peak detection to determine the frequency offset between the local oscillation and the received RF signal, and makes a corresponding adjustment to local oscillation based thereon. However, this conventional method for detecting frequency offset is prone to errors, largely because the peak detection period is defined without the knowledge of the particular data pattern being received. Thus, frequency deviations intended to indicate a binary “1” or “0” may be misinterpreted to be a frequency offset of the center frequency.
For instance, if a long data string of 0's or a long string of l's is received, the opposite peak can be correctly detected, causing the frequency offset to be erroneously calculated, resulting in incorrect adjustment of local oscillation. Moreover, intersymbol interference may cause the peaks to appear differently depending on the particular data patterns. Thus, if the opposite peaks are determined based on a data signal containing various interference effects, the detected offset will likely be inaccurate.
There is a need for an accurate apparatus and technique for providing fine automatic frequency compensation, particularly in time-sensitive applications and/or in the presence of intersymbol interference.